The problems of soaring energy demand and environmental pollution are addressed by various biological processes for the treatment of industrial wastes. Biohydrogen production through dark fermentation is one known process for the treatment of industrial waste for the production of hydrogen.
Microorganisms are capable of producing hydrogen via either photosynthesis or preferably through fermentation (Matsunaga, T., Hatano, T., Yamada, A., Matsumoto, M., (2000) Microaerobic hydrogen production by photosythetic bacteria in a double phase photobioreactor. Biotechnol. Bioeng. 68 (6), 647-651). Organic pollutants are anaerobically converted to methane in two distinct stages: acidification and methanogenesis. Acidification produces hydrogen as a by-product which in turn is used as an electron donor by many methanogens at the second stage of the process (Fang, H. H. P. and Liu, H. (2002) Effect of pH on hydrogen production from glucose by a mixed culture. Bioresource Technology 82, 87-93). Separation of the two stages is feasible for hydrogen collection from the first stage. The second stage is further used for treatment of the remaining acidification products, which includes mainly volatile fatty acids (VFAs).
The continuously stirred tank reactor (CSTR) has been the most widely used system for continuous hydrogen production (Li, C., Fang, H. H. P., (2007) Fermentative hydrogen production from wastewater and solid wastes by mixed cultures. Critical reviews in Env. Sci. and Tech., 37, 1-39). Since in a CSTR biomass solids residence time (SRT) is the same as the hydraulic retention time (HRT), its concentration in the mixed liquor is highly affected by the recommended HRT of 1-12 h which is optimal for high hydrogen production rates (Li and Fang, 2007). The maximum specific growth rate (pmax) for mixed culture of 0.333 h-1 (Horiuchi J. I., Shimizu T., Tada K., Kanno T., Kobayashi M., (2002) Selective production of organic acids in anaerobic acid reactor by pH control. Bioresource Technol 82, 209-13) corresponds to an SRTmin of 3.0 h.
However, high dilution rates result in a marked decrease in biomass content in the reactor due to severe cell washout and system failure (Wu, S. Y., Hung, C. H., Lin, C. Y., Lin, P. J., Lee, K. S., Lin, C. N., Chang, F. Y. And Chang, J. S. (2008) HRT-dependent hydrogen production and bacterial community structure of mixed anaerobic microflora in suspended, granular and immobilized sludge systems using glucose as the carbon substrate. Int. J. Hydrogen Energy 33, 1542-1549). Since Acetone-butanol-ethanol (ABE) fermentation utilizes the same bacterial groups that are used for biohydrogen, the process also suffers from biomass washout. Therefore, to resolve biomass washout in ABE fermentation, most of studies in the literature and full-scale applications have utilized batch or fed-batch reactors.
Decoupling of SRT from HRT in hydrogen bioreactors has been achieved primarily by using biofilms on several media including synthetic plastic media and treated anaerobic granular sludge (Das, D., Khanna, N., Veziroglu, T. N., (2008) Recent developments in biological hydrogen production processes. Chem Ind. And chem. Eng. 14 (2), 57-67), activated carbon, expanded clay and loofah sponge (Chang, J. S., Lee, K. S., and Lin, P. J., (2002) Biohydrogen production with fixed-bed bioreactors. Int. J. Hydrogen Energy 27 (11/12), 1167-1174), glass beads (Zhang, H., Mary, A. B., Bruce, E. L., (2006) Biological hydrogen production by clostridium acetobutylicum in an unsaturated flow reactor. Water Research 40, 728-734) and membranes (Vallero, M. V. G., Lettinga, G., and Lens, P. N. L., (2005) High rate sulfate reduction in a submerged anaerobic membrane bioreactor (SAMBaR) at high alinity. J. Membr. Sci. 253(1/2), 217-232). Problems with the development of methanogenic biofilms on the carrier media adversely impact process stability, which is critical for sustained hydrogen production. Moreover, membranes have not shown many advantages in terms of volumetric hydrogen yield and are also prone to fouling in such a reductive environment.
A biohydrogenator system provided in WO2010/085893 is intended to address two limitations for sustained biological hydrogen production: contamination of the microbial hydrogen-producing cultures with methane-producing cultures and low bacterial yield of hydrogen-producers. In that system, a gravity settler is used after a hydrogen reactor for decoupling SRT from HRT through sludge. The system disclosed includes a CSTR for biological hydrogen production, followed by a gravity settler positioned downstream of the CSTR, the combination of which forms the biohydrogenator. The biomass concentration in the hydrogen reactor is kept at the desired range through biomass recirculation from the bottom of the gravity settler and/or biomass wastage from the gravity settler's underflow. This prior art biohydrogenator is described to increase hydrogen yield from sugar and carbohydrate based wastes from 1.6 to 3.2 mol H2/mol glucose while producing VFAs primarily acetate as the residual soluble metabolite. Although that represents an improvement over previous systems, this biohydrogenator is still subject to the limitations common to dark fermentation processes: the inhibition of hydrogen production by the accumulation of fermentation end-products. The production and accumulation of acetic and butyric acids results in lower hydrogen yields and a total undissociated acid concentration of 19 mM initiated solventogenesis. Different strains of Clostridium produce different ratios of end-products thus affecting their hydrogen-producing potential. The elimination of butyric acid formation and the increased production of acetic acid would provide for increased hydrogen yield from glucose. Although acetate production would increase hydrogen yield to 4 mol of hydrogen per mole of glucose, this is still not enough for the process to be an economically viable alternative to existing hydrogen production methods.
Another biohydrogen production process is the electrohydrogenesis process. In an electrogenesis process, exoelectrogenic bacteria are able to release electrons exogeneously (outside the cell) to solid substrates (i.e. a carbon electrode), allowing electricity to be produced in a reactor called a microbial fuel cell (MFC). The oxidation reaction generated by the bacteria at the anode is sustained through the production of water at the cathode from electrons and protons released by the bacteria, and oxygen. The electrohydrogenesis process is similar except that a small potential must be added into the circuit and no oxygen is used at the cathode. Thus, hydrogen gas is evolved at the cathode in a reactor called a microbial electrolysis cell (MEC). The process has also been referred to as a bacterial electrolysis cell (BEC) and a bioelectrochemically assisted microbial reactor (BEAMR): Liu, H., Grot, S., Logan, B. E., (2005) Electrochemically assisted microbial production of hydrogen from acetate. Environ. Sci. Technol., 39, 4317-4320; Rozendal, R. A., Buisman, C. J. N., Bio-Electrochemical Process for Producing Hydrogen. International Publication No. WO 2005/005981; and Rozendal, R. A., Hamelers, H. V. M., Euverink, G. J. W., Metz, S. J.; Buisman, C. J. N. (2006), Principle and perspectives of hydrogen production through biocatalyzed electrolysis. Int. J. Hydrogen Energy, 31, 1632-1640. The BEAMR process differs from MFC with respect to loss of hydrogen due to its diffusion from the cathode chamber through the cation exchange membrane (CEM) into the anode chamber. In addition, in the BEAMR process there is no potential for loss of substrate resulting from aerobic growth of bacteria due to oxygen diffusion into the anode chamber from the cathode chamber.
All of these known electrohydrogenesis processes are coupled with, and in fact dependent on, an upstream dark fermentation process (i.e. two completely separate process stages). The advantages of a separate electrohydrogenesis process were evaluated in Liu et al. (2005), Rozendal & Buisman (2005), and Rozendal et al. (2006).
The most widely reported problems with dark fermentation reactors in the literature were microbial shifts, metabolic shifts, biomass washouts, repeated systems failure, sustainability, low hydrogen yields, methanogens growth and methane production as opposed to hydrogen. With all the aforementioned problems, researchers have moved on to the MEC microbial electrolysis cells and assumed that in the future a sustainable dark fermentation process can be used upstream of the MEC for acids production in dark fermentation to be utilized in the MEC. Again with the MEC, researchers have reported several problems due to their reactor configurations i.e. high hydrogen yields requires short retention time in the MEC while the microorganisms tend to escape at high dilution rates thus researchers always use long retention times in the MECs in an attempt to mitigate biomass washout, but extending retention times lead to the growth of methanogens, a group of microorganisms which consumes hydrogen and produces methane. Thus, an improved process is desired which would address at least some of these problems.